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It began back in 1994 with a short note in the journal Nature, about a curious bacterium from the Aberjona watershed, strain MIT-13, that could grow on arsenic (1). Arsenic resistance had been well established, as it had been found in many clinical species like Staphylococcus aureus and Escherichia coli. Arsenite oxidation linked to resistance was also known. However, this was different—the organism, later to be named Sulfurospirillum arsenophilum (2), could couple the oxidation of lactate to the reduction of arsenate [As(V)] for growth. More significantly, incubation experiments revealed dissolution and reduction of arsenic from the sediments. This suggested that such organisms could be responsible for mobilizing arsenic in aquifers, resulting in the poisoning of millions of people worldwide. As more researchers began investigating microbial arsenic metabolism, it became apparent that not only was there a robust biogeochemical cycle, but arsenic played a role in the evolution of life on Earth (3). Thus began the journey to decipher how these organisms were capable of harnessing the energy from arsenic oxyanions. Now, Glasser et al. (4) have found the holy grail by solving the crystal structure of the respiratory arsenate reductase, Arr, from Shewanella sp. ANA-3. In doing so, they answer several questions, including the type of iron sulfur clusters in both ArrA and ArrB, the coordinating ligand to the molybdenum (cysteine, as had been predicted), the nature of the catalytic pocket, the binding of arsenic to the reaction site, and the mechanism of substrate transformation. Furthermore, they propose a reasonable solution to the preferred electron flow in these bidirectional enzymes.

The respiratory arsenate reductase, Arr, was established both biochemically and through gene knockout studies (5⇓–7). Once key conserved regions in the protein sequence were recognized, more examples were found in both Bacteria and Archaea. Although …

↵1To whom correspondence may be addressed. Email: stolz{at}duq.edu or basup{at}iupui.edu.

Bacteria could help tackle the growing mountains of e-waste that plague the planet. Although researchers are a long way from optimizing the approach, some are already confident enough to pursue commercial ventures.

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